Assignment 8: Sokoban

Transcription

1 Assignment 8: Sokoban Algorithmic Thinking and Structured Programming (in Greenfoot) c 2015 Renske Smetsers-Weeda & Sjaak Smetsers Licensed under the Creative Commons Attribution 4.0 license, 1 Introduction In this assignment you will write a game in Greenfoot known as Sokoban. See info/ for an example. Our variation is called Mimi the eggs-hoarder. The original game takes place in a warehouse. The player is a warehouse employee whose task is to push crates to the correct location. This game is played in the Madagaskar world; the world in which Mimi lives. The goal is for Mimi to push all the eggs into the nests. The following rules apply: Mimi can only push eggs forward, she can t pull an egg; Mimi cannot sit on an egg or step over it; Mimi can only push one egg at a time. As a consequence, Mimi cannot push two (consecutive or adjacent) eggs simultaneously; No two eggs can occupy one cell (not even in a nest); A nest can hold only one egg; An egg can be pushed out of a nest; Neither Mimi nor an egg can be pushed through a fence; Mimi can step over a nest (of course, very carefully so that she doesn t destroy it). If the nest holds an egg, she will push the egg out; There are as many nests in the world as eggs; The level is completed when each nest is filled with an egg.

2 2 Learning objectives After completing this assignment, you will be able to: handle user interaction in the code; apply nested if.. then.. else statements; apply the knowledge from previous assignments to implement a game on your own. 3 Instructions For this assignment you will need the DodoScenarioSokoban scenario. 4 Theory Contrary to the previous assignments, you will not implement an algorithm to make Mimi do something. This time the user directs Mimi by pressing a key, and in the code we must explain what Mimi must depending on which key is pressed. We start by explaining a particular use of if.. then.. else statements, this time in the context of Sokoban. This is needed to incorporate user-interaction for the different keys. Nested if.. then.. else.. statements You can test multiple cases simultaneously using a nested if.. then.. else statement. This was explained in assignment 7, using a general example. We will now give a Greenfoot example that will also be used in this assignment. Example: Suppose we want to check whether the user has pressed one of the arrow keys on the keyboard, and if so, have Mimi face that direction. Code using nested if.. then.. else.. statements The Greenfoot method boolean iskeydown (String key) tests whether the user has pressed a particular key. For example, to check whether the arrow-up button has pressed, the following method is called: Greenfoot. iskeydown ("up" ) ; Similarly, the string "left" is associated with left-arrow-key, "right" with the right-arrow-key, and "down" with the down-arrow key. In the handlekeypress method this test is used to determine which key has been pressed and changes the direction in which Mimi is facing accordingly: public void handlekeypress( ) { if ( Greenfoot. iskeydown( "left ") ) { setdirection ( WEST ) ; else { if ( Greenfoot. iskeydown( "right" ) ) { setdirection ( EAST ) ; else { if ( Greenfoot. iskeydown( "up" ) ) { setdirection ( NORTH ) ; Algorithmic thinking and structured programming (in Greenfoot) 2

3 else { if ( Greenfoot. iskeydown( "down" ) ) { setdirection ( SOUTH ) ; Flowchart using nested conditional statements: The corresponding flowchart looks like this: Figure 1: Flowchart for handlekeypress Note: As a result of the numerous accolades and the fact that the code stretches further and further to the right, the code becomes hard to oversee and furthermore being errorprone. Code using else.. if.. statements In Java, there is an alternative way for writing these types of nested conditional statements. Most programmers combine the else and the if into one code-line using an else.. if.. statement. The method handlekeypress then becomes: public void handlekeypress( ) { if ( Greenfoot. iskeydown( "left" ) ) { setdirection ( WEST ) ; else if ( Greenfoot. iskeydown( "right" ) ) { setdirection ( EAST ) ; else if ( Greenfoot. iskeydown( "up" ) ) { setdirection ( NORTH ) ; else if ( Greenfoot. iskeydown( "down" ) ) { setdirection ( DOWN ) ; Note: This results in less lines of code, and furthermore, the code-structure becomes much clearer. Algorithmic thinking and structured programming (in Greenfoot) 3

4 5 Exercises (implementation of the game) For this assignment you will start with a brand new scenario. Compared to the previous scenarios, some minor modifications have been made. The most important addition is the MovableActor class. Each Dodo can be moved (and is thus movable), and now belongs to the MovableActor class. Contrary to the previous assignments, Eggs can now be moved too. As a consequence, Eggs now belong to the MovableActor class too. The class diagram shows both Dodo and Egg as sub-classes of MovableActor. The MovableActor class basically has no new functionality. The main difference is that several move -related methods such as void step ( int direction) and boolean borderahead (int direction) have been moved from the Dodo class to the MovableActor class. Mimi now inherits these methods from MovableActor. See figure 2: Figure 2: Methods which Dodo inherits from MovableActor Because Eggs now also belong to the MovableActor class, they too can use the step and borderahead methods. In addition, several new methods have been added to the Egg class. The most important are: void push ( int direction): which allows an egg to be pushed in a particular direction. This direction is passed as a parameter. boolean canbepushed ( int direction ): which can be used to test if an egg can be moved in the specified direction. 5.1 Getting started 1. Download and open the DodoScenarioSokoban scenario; 2. Compile the scenario and press Run. Mimi should run along the inside of the fence that encloses the world. 3. Using the right-mouse-button, test the new Egg methods push and canbepushed several times, each time with the egg in a different position in the world. Do these methods work as expected? 4. Replace the code in MyDodo s act method by a call of the method handlekeypress. 5. Compile and run the scenario (by pressing the Run button). Desbribe what happens. If nothing happens, then press one of the arrow keys on your keyboard. As you can see the handlekeypress method is not complete yet. Which key(s) does Mimi respond to? Which doesn t Mimi respond to. 5.2 Mimi responds to arrow keys As you may see, lots of things don t work properly yet. Mimi walks through fences and over eggs, and does not adequately respond to the arrow keys pressed by the user. In this task you will create order in Mimi s world. We start by fixing Mimi s behavior to the user s instructions: Algorithmic thinking and structured programming (in Greenfoot) 4

5 1. Open the MyDodo class in the editor and find handlekeypress. 2. Have a look at the body of the handlekeypress method. Here, a submethod called getnewdirection is used. 3. Consider where you have to change which code so that Mimi adequately responds to all the arrow keys. Tip: Have a look at the theory block above about Nested if.. then.. else.. statements in which the Greenfoot method iskeydown is discussed. 4. Adjust and compile MyDodo. 5. Run the scenario and make sure Mimi indeed responds to each arrow key adequately. If not, adjust your code. 5.3 Mimi pushes eggs forward (A) Mimi takes a step when the user presses an arrow button. However, if Mimi is in front of an egg, she shouldn t merely take a step. More must happen! We will now ensure that Mimi doesn t just step over an egg, but that she pushes it forward. We will now adjust handlekeypress to work as expected. To do so, follow the next tips: First decide which specific cases should be distinguished and adjusted; The Dodo class contains all the methods which you need to distinguish these cases; Obviously, the new Egg methods will be useful; Mimi can check whether there is an egg laying directly in front of by calling eggahead; Mimi can get a hold of the egg using geteggahead; Using the existing methods, she can ask the egg whether or not it can be pushed, and if so, push it forwards and then (in the same direction), take a step herself. Compile and test your changes. Systematically try a few cases. Fix any errors you encounter. Make sure that Mimi indeed complies to all the rules described in chapter Scoreboard We need a scoreboard in order to track how many steps Mimi has taken and how many eggs she has already placed in a nest. We will let Mimi keep track of these scores herself and have a scoreboard display the values. Do this as follows: 1. First, add two instance variables to MyDodo for storing this information (one for number of steps taken, the other for the number of eggs in a nest). Consider meaningful names, appropriate types and a suitable initial value. 2. As soon as Mimi takes a step, change the value of the instance variable for the number of steps taken. 3. After each step, call updatescores to ensure that the changed situation is actually displayed on the scoreboard. 4. To ensure you haven t made any mistakes thus far, compile and test your changes before proceeding. 5. Now add the second variable to keep track of how many eggs have been placed in a nest. Consider when and how you can determine this. Tip: The Dodo method boolean nestahead() may be useful. Algorithmic thinking and structured programming (in Greenfoot) 5

6 6. Again, call updatescore. 7. Compile and test your program. 5.5 Egg in its nest At the time-being, when an egg is pushed into a nest, you can t see it anymore. This is because the egg and the nest are in the same cell, and the egg is hidden behind the nest. That s too bad! Let s change that. So, if an egg is pushed into a nest, we want to show a nest with an egg in it. To do this, we first adjust the Nest class. 1. Open the Nest class in the editor. 2. Add an instance variable to this class which indicates whether the nest is empty or not. Ensure that this variable gets an appropriate initial value (i.e. when a new nest is brought into the world, what should its value be?). 3. Add a method to fill the nest when appropriate. The effect of this is not only to change the value of the instance variable, but also to adjust the picture which is shown. Tip: Look back to assignment 6 To be hatched or not to be hatched in which you did something similar. There, in the class Egg, you also changed the picture of an egg into a hatched egg. 4. Compile and test your program using your right-mouse-button. 5. Similarly, add a method to empty the nest when an egg is pushed out. 6. Compile and test your program using your right-mouse-button. 7. Also adjust your MyDodo class. First consider what exactly must happen and where your code needs to be adjusted. Recall that the nest itself does not know whether or not it contains an egg; Mimi, however, does! To change the state of the nest, you need (a reference to) the nest object. Which method can you use to do this? Keep in mind that you can also push an egg out of a nest. 8. Compile and test your program, this time also run the scenario. 5.6 Level achieved (A) The program must now decide whether the level has been completed or not. We introduce a referee to make this decision. 1. Have a look at the code of the Referee class. 2. At the top of the class an instance variable myworld is declared which references to the world. Further down, you can find a method updatescoreboard. This method is used in the Dodo class and wille called as soon as updatescores adjusts the score. If you wish, you can check this yourself by opening the Dodo class in the editor and having a look at the updatescores method. 3. The second score, called score2 indicates how many eggs are already in a nest. If this is equal to the number of eggs in the world, the level has been completed. Add a method that checks whether the game is over. If this is true, call the method levelfinished. Tip: Use the World method getobjects to determine how many eggs the world contains. Have a look at the Java documentation about what the method does and consider how you can use the result of this method to determine if the level has been completed or not. Algorithmic thinking and structured programming (in Greenfoot) 6

7 5.7 Optional: Oops.. undo? Finally, it is still a bit unsatisfying that when the user accidentally presses the wrong key he may not be able to complete the level anymore (he gets into a situation in which the level cannot longer be solved). An undo function would be helpful. In order to achieve this, some code adjustments must be made in the mydodo class. We will only give some general indications on how to do this; its up to you to come up with and implement the details. 1. Determine a suitable key for an undo. Add a method to handle the user-interaction. 2. Determine which information must be remembered/stored in order to undo a move. 3. Besides having Mimi take a step back, sometimes an egg s position must be restored (or undone) too. 4. Consider what should happen if the user wants to undo multiple steps. 5. Does the correct value appear on the scoreboard after an undo? 5.8 Optional: New levels Add your own new challenging levels to the scenario. Let a fellow student test whether they are solvable or not. Also have them decide what the level s difficulty is. 6 Summary You can now: handle user-interaction (keyboard input); make use of else if statements; use what you have learned in previous assignments to write a game on your own. 7 Saving your work You have just finished the bonus assignment. Save your work! 8 Handing in Hand your (digital) work in via to renske.weeda@gmail.com: Algorithmic thinking and structured programming (in Greenfoot) 7